I would argue that it's a sad ending in the original for when Marty goes back to 1985 (after saving the day). He wakes up and the people in his family are the same but they've changed into totally different people. He has no idea/memories of his family's past to get to that point, so it does seem like Marty is isolated because he's only know the 'loser' version of his family (before he went back to 1955).

I wouldn't classify it as a sad ending however. I would say it's either a plot hole or suspension of disbelief depending on how much of an issue it is to the viewer. It was written to be "happy" which is why I say that.

-Marshall-
Nun sacciu, nun vidi, nun ceru e si ceru durmiv.I know nothing, I see nothing, I wasn't there,
and if I was there, I was asleep.

I think it was overthinkingit.com that suggested that BTTF's sad ending was because Marty would be interrogated and imprisoned by the FBI because Doc Brown was missing with plutonium and there was a van full of dead terrorists at the mall.

mavrach wrote:I think it was overthinkingit.com that suggested that BTTF's sad ending was because Marty would be interrogated and imprisoned by the FBI because Doc Brown was missing with plutonium and there was a van full of dead terrorists at the mall.

I'm not sure but I assumed that never happened. It's one of the wonders of a time paradox but if he changed his life then he would have changed the need to go back in time so none of it would have happened like that. Hence the paradox. Kind of like going back in time to kill yourself. If you did that you wouldn't be alive in the future to do it. However that is an interesting thought. We are most certainly overthinking it at this point however and my brain is starting to hurt.

Andrew, let's go to the parts store instead. I need some shiny chrome.

-Marshall-
Nun sacciu, nun vidi, nun ceru e si ceru durmiv.I know nothing, I see nothing, I wasn't there,
and if I was there, I was asleep.

The future's what you make of it. Turns out, so is the past. And the present...

It's been awhile, but I'm pretty sure I remember Marty going back at the end of the first film and watching the terrorists chase his past self, and then going to the shot-up Doc, so it looks like it did all happen.

I'm just a horse, standing in front of a boy, asking him to love me. - Andrew Forbes

mavrach wrote:I think it was overthinkingit.com that suggested that BTTF's sad ending was because Marty would be interrogated and imprisoned by the FBI because Doc Brown was missing with plutonium and there was a van full of dead terrorists at the mall.

Don't worry; UNIT and the Celestial Intervention Agency moved in and covered the whole thing up.